To increase the running stability of a vehicle, it is desired that the condition of a road surface on which a vehicle is running, or the coefficient of friction between tire and road surface (road surface friction coefficient), be estimated with accuracy and the estimated result fed back to vehicle control. In particular if the road surface condition, or the value of road surface friction coefficient, can be estimated before the driver initiates a danger-avoiding control, such as braking or steering, it will help enhance the accuracy of vehicle control technology, such as ABS (anti-braking system) and VSC (vehicle stability control), thus improving vehicular safety markedly.
As a conventionally proposed method for estimating a road surface friction coefficient, there is a technique for estimating a road surface condition, especially a maximum friction coefficient of a road surface, from a relationship between the change in slip ratio when the accelerator or brake is operated and the vehicle body acceleration. This method utilizes the correspondence between the measure of road surface friction coefficient μ and the vehicle body acceleration Ab. That is, the condition of a road surface when a vehicle is running is estimated by comparing a vehicle body acceleration with the predetermined Ab/S values for a vehicle running on a low μ road, an medium μ road, and a high μ road, within a stable domain of a “vehicle body acceleration Ab−wheel slip S” characteristic curve. By this method, the measure of road surface friction coefficient μ can be easily estimated from the vehicle body acceleration Ab (See Reference 1, for instance).
There is also a proposed method which utilizes the fact that the vibration level of a tire of a running vehicle changes with the road surface condition. According to the method, a vibration sensor or the like is attached to a tire and a road surface condition is estimated using the tire as a sensor. In this method, a vibration sensor is installed on the inner side of a tire tread, and the vibration level of the tire tread portion of a running vehicle is detected. Then after obtaining a vibration waveform of the vibration level arranged in a time series, a curve representing a vibration level distribution is prepared by correlating the vibration detecting positions to the time axis of the waveform and having the vertical axis represent the power value (OA, or oscillation amplitude, power value of vibration). Thus a road surface condition while a vehicle is running is estimated by comparing an OA power value of vibration in a tire contact patch domain of this vibration level distribution with a previously prepared master curve of vibration level distribution for the running of a vehicle on various kinds of road surfaces. This method enables accurate estimation of a road surface condition while the vehicle is running (See Reference 2, for instance).
Reference 1: Japanese Unexamined Patent Application Publication No. 7-112659
Reference 2: WO 01/098123 A1